1. Field of the Invention
This invention relates to digital sampling of analog communications signals, and more particularly to receiver-based sampling error elimination in video transmission systems having a specified number of intensity level time intervals per horizontal sweep.
2. Description of the Prior Art
In the continuing search for more reliable video transmission systems, several different approaches have been tried. One useful approach has been to make use of a process of sampling and quantizing, which comprises converting continuous signals of time and amplitude into discrete quantized "stairsteps" uniformly sequenced in time. As described in U.S. Pat. No. 2,681,385, issued to B. M. Oliver on June 15, 1954, sampling alone does not necessarily entail loss of significant information as long as the sampling frequency is at least twice as great as the highest frequency of interest in the information signal. Furthermore, quantization does not cause a serious loss of information if the number of quantizing levels is sufficiently high.
The quantized signal thus obtained may be, if desired, stored in the form of individual digital bytes in a digital memory. Information thus stored may be conveniently retrieved from the digital memory whenever needed. Such a system typically employs eight bit digital data bytes for storage, thus allowing for 2.sup.8 =256 quantizing levels (which number is more than adequate for most applications).
Oftentimes it is necessary or desirable to transmit such stored video data along an analog transmission channel (e.g. a coaxial cable or a radio link). In any case, eventually the digital data will need to be reconverted to an analog video signal in order to display it on an analog display device. To do so, the digital memory is interrogated sequentially and the data is processed through a digital-to-analog (D/A) converter to recreate an analog stairstep video signal.
Reception of such a signal is straightforward and well known, but properly sapling it for later quantized data storage in a digital format can be quite difficult. The main problem with such sampling lies in avoiding sampling during the transition times between sample levels, i.e. during the almost vertical portions of the stairstep waveform; sampling therein results in faulty data being stored and a corresponding serious degradation of a visual image produced therefrom. In particular, direct sampling of such a waveform at a fixed rate often results in the well-known "aliasing" phenomenon which evidences itself as "beat patterns" (i.e., alternate light and dark regions) in the visual image produced. Sampling at any rate not an exact integer multiple or subharmonic of the transmitted rate will result in a variation in the number of samples per pixel, creating a spatial distortion of the data in the receiver's memory.
One traditional approach to eliminate such beat patterns involves passing the received stairstep analog signal through a low-pass filter prior to sampling the signal in order to broaden the transition times between successive sample levels. Such an approach, used judiciously, does serve to reduce substantially the observable beat patterns; unfortunately, that approach also invariably removes frequency components of the signal near the pixel rate which reduces the overall sharpness or "definition" of the visual image produced. This is somewhat analogous to adjusting the treble control on a car radio to eliminate high frequency static -- the static goes, but so do the cymbals-
It is therefore desirable to avoid the above-described prefiltering step if possible. Another approach to the problem is to oversample the received signal. This works reasonably well, but it requires significantly expanded storage space for the data thus collected. In order to eliminate aliasing without prefiltering, and without oversampling, it is necessary to keep the receiving sampling system in step with the transmitter.
Several approaches have been employed in the prior art, each with the express goal of sampling a video signal accurately, to wit:
(1) U.S. Pat. No. 3,971,063, issued to P. C. Michael, et al. on July 20, 1976, describes a system for compensating for potential variations in timing between horizontal synch pulses of a received video signal which might occur due to undesired perturbations (e.g. wow and flutter) in the tape transport speed of a transmitting video tape recorder. Therein an error signal is generated by comparing the time to produce the receiver's normal number of sampling clock pulses per line with the actual time period between each of the transmitted horizontal sync pulses. The error signal thus generated is employed to vary the period of the normal number of sampling clock pulses in sympathy with the fluctuating off-tape transmitted line periods between horizontal sync pulses to maintain a constant difference between them. No fixed number of transmitter pixel clock pulses per horizontal sweep is used, and no synchronized pixel-by-pixel sampling is accomplished thereby.
(2) U.S. Pat. No. 4,105,946, issued to Y. Ikeda on Aug. 8, 1978, discloses a phase-locked-loop frequency synthesizer that employs a digital counter to determine whether or not a slave oscillator is operating at a desired harmonic frequency with respect to the frequency of a reference oscillator. An error signal corresponding to the magnitude of a differential count is generated and used as the control voltage for the phase locked loop. Direct access is made therein to the source reference oscillator, and no synchronization is needed or attempted.
(3) U.S. Pat. No. 4,613,827, issued to T. Takamori, et al. on Sep. 23, 1986, teaches a phase locked loop approach to generating a frequency that varies as a function of the time difference between the horizontal sync pulses of an input video signal from a video tape recorder, said frequency being held in locked phase with the color burst signal and horizontal sync signal portions of said input video signal. The purpose of the system therein described is to remove jitter from a tape-recorder-produced video signal. The system of Takamori does not employ a bang-bang anti-aliasing feedback system comparable to that of the instant invention.
(4) The article entitled Burst-Locked Oscillator Avoids Side Lock, which appeared in NASA Tech Briefs, May, 1988, page 20, protrays a digital error-detection-and-correction scheme for a color-television oscillator circuit that stabilizes the receiver color burst frequency. The system employs a read-only-memory (ROM) controller that provides a high or low oscillator frequency correction control signal in response to the output of a counter. The counter counts the number of cycles of a voltage controlled oscillator that occur in each picture line (i.e., the horizontal data portion of the received video signal). This system allows the oscillator to vary as much as one full oscillator cycle per picture line while still being deemed to be the correct number of cycles per line. As such the oscillator would not be suitable for use with a synchronized pixel-by-pixel data sampler in accordance with Applicant's novel approach hereinafter disclosed and claimed.
None of the approaches described above makes full use of a significant fact that is known a priori for most stairstep video signal transmitting and receiving systems, i.e., the number of stairstep periods (also known as pixel clock periods) per each horizontal sweep data portion of the video signal is a known integer constant. It is therefore an object of the instant invention to provide an improved method and apparatus for sampling a received stairstep video signal very accurately and efficiently by requiring that the number of receiver and sampler pixel clock periods per each horizontal sweep data portion of the received video signal is maintained within a range of values that is very close to a predetermined constant.